Sunday, February 10, 2008

5. A Study in Camera Compensation when using Flash in TTL Mode

The SB800 Flash Instruction Manual says that when using the flash in TTL mode, adjusting Camera Compensation will lighten or darken the entire image, which means it increases or decreases flash power along with the ambient image, so I thought I examine that subject.

Conditions:

Normal indoor ambient lighting

Subject five feet away is a small statue of a boy on a horse placed in the center of the frame to be sure the flash metering will measure it properly

Background is a bookcase 10 feet away

Camera in Matrix metering

Flash in TTL mode

Note: The images in this study are directly from the D200 camera without any modification except compression to make them suitable for uploading.
Image 1: Flash OFF, Camera A mode, 1/25th, f/2.8, Camera ev=0I shot Image 1 with the flash Off using camera A mode with f/2.8 selected. I could see in the viewfinder that the camera automatically selected 1/25 shutter. Notice that the image is properly exposed with pixels in the histogram extending all the way from black shadows on the left to bright highlights on the right and there are some shadows on the face and is approximately the same brightness as the background. The yellow color is due to the incandescent lighting and the poor Auto White Balance function on the D200. I will not try to correct or balance the color during this study.

Image 2: Flash On in TTL mode, FEC=0, Camera A mode, 1/60, f/2.8, Camera ev=0 Note: FEC means Flash Exposure CompensationIn Image 2 the flash was used, and as soon as the flash was turned ON, the shutter changed to 1/60th, which is the lowest available flash shutter setting as determined by CSM e2. Note that the faster shutter darkened the background a bit compared to Image 1, which is to be expected. You can see how the dark pixels are bunched up against the left edge of the histogram, meaning that detail is being lost in the shadows. The hump of pixels on the right come from the subject. Note that the histogram shows that there are no blown out pixels (ie, climbing up the right edge), but the subject still looks too bright for the scene. This happened because the flash metering system measures a little too 'hot' on my D200/SB800 combination. I think this is a very common thing on the D200/SB800 combinations. That's why I normally dial in -.7 ev on the flash for indoor shots (but this is a sidebar and not the main subject of this study).
Notice also that the color of the bookcase in the background is still yellow since it is lighted mainly by the ambient incandescent lights while the subject is much whiter since it is lighted mainly by the flash.
Image 3: Flash ON in TTL mode, FEC=0, Camera A mode, 1/100th, f/2.8, Camera ev = -2In Image 3 I decreased the camera EV to -2.0. You can see that both the background and the subject got darker (just like the SB800 book says they should). You can also see that the hump of pixels on the right moved left just like the darker pixels on the left, piling more of them up on the left edge. The question I am studying is exactly how did the camera & flash achieve this left shift of the pixels?
Well, I observed that the shutter automatically increased to 1/100, which darkened the background. You would have expected that when starting at 1/60th it would have increased to 1/240th to achieve two stops darker. However, I have discovered that the camera knew the ambient was actually at 1/25, and 1/100 is exactly two stops darker than 1/25. Pretty smart system.
Remember that changing shutter speed does not affect the flash portion of the exposure, because the flash occurs is so short (less than 1/1000th second).
And you can see that changing the camera ev definitely darkens the subject as well, which means it has to decrease the flash power. That indicates that the camera compensation information is sent back to the flash metering system when shooting in TTL mode.
So, when you change the camera ev, it changes the camera f/ stop or shutter speed settings to adjust the background, and it sends the ev change to the flash to change the flash ev by that same amount to adjust the subject.

Image 4: Flash OFF, Camera M mode, 1/25th, f/2.8, Camera ev = 0To further verify exactly what the camera ev settings tell the flash, I first put the camera in Manual mode and set 1/25 and f/2.8 and shot Image 4 above without flash as a reference. Note that Image 4 and its histogram looks essentially identical to Image 1, as it should, since the same camera settings were used.

Image 5. Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev=0Then in Image 5 I increased the shutter to 1/60th and fired the flash, and this image looks just Image 2, as it should.

Image 6: Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev = -2Now, in Image 6, with the camera in Manual mode, 1/60th, f/2.8, you can see the effect of setting camera ev to -2. Comparing with Image 5, the subject in Image 6 definitely got darker, and the histogram hump on the right moved slightly left, but the background didn't change. This proves that ev changes on the camera are sent to the flash to the change its power, and the reason the background didn't change is because ev changes to the camera will not affect the f/ stop and shutter when in camera Manual mode. In the previous example, the camera was in A mode, and camera ev changes will change the shutter speed.

Image 7: Flash OFF, Camera M mode, 1/250th, f/2.8, Camera ev= 0
I took this shot without flash as a reference image to show just how little ambient there is at 1/250th. 1/250th virtually takes the ambient light out of the exposure in normal indoor lighting.

Image 8: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, Camera ev=0In Image 8 I turned on the flash and took the shot. Notice the dark background, because the flash exposes the subject correctly, but only a small amount reaches the background five feet behind the subject (due to the square law - power decreases by the square of the distance).

Image 9: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, f/2.8, Camera ev = -2Then, in Image 9 camera ev was reduced by 2 ev and the brightness of the subject (power of the flash) definitely decreased compared to Image 8. The brightness of the background barely changed, though because not much of the flash power is getting back there. This is showing that you can adjust the brightness of the subject without changing the background by changing flash power.

Image 10: Flash On in TTL mode, FEC = -2, Camera M mode, 1/250th, f/2.8, Camera ev = 0Lastly, in Image 10 I set the camera at 1/250, f/2.8, and ev 0, and I used the Flash ev on the back of the flash to -2 ev. As you can see, the image looks exactly like Image 9, and the hump of pixels on the right (representing the subject) is in the same spot on the histogram.Conclusion:

This study proves conclusively that camera ev is sent to the flash in TTL mode and can be used to adjust flash power, just like the FEC button on the front of the camera, and the FEC button on the flash itself.

Great article & great web site. This cleared a few things up in my mind.

I would be grateful if you could clarify one thing: When in manual, are the EV and the FEV settings combined. For example, if EV is set to +3 and FEV is set to +1 will this mean, in practice, a cumulative increase of +4, or does one cancel the other out?

In TTL mode only, the camera EV and the Flash EV are cumulative on the flash setting but not on the camera exposure.

In other words, +2ev on the camera and +1 flash EV will result in the flash at +3 EV. However the Flash EV doesn't add to the camera exposure. So, in this example, the camera will stay at +2ev and the flash will be at +3 ev.

TTL-BL works differently, and somewhat unpredictably, so I never use camera ev when using TTL-BL unless I feel adventuresome :)

I'm an D80 owner and trying to follow by your steps here. My investigation is stopped at Image3 when TTL flash is ON, with camera in A-mode and EV=-2. Though the picture becomes darker (as it should be), the shutter remains the same as with EV=0 (1/60).So in my case the negative compensations lowers the flash power only, but how about the shutter? From this one could conclude that FEV and EV do the same thing, but this is VERY, VERY strange...

Is it an D80 issue or there is something wrong with camera's settings? Or something else? Could you tell?

And again, let me acknowledge my appreciation and many thanks for the great job you are doing here.BR,j0t.

I think that in your case, the ambient light must have been dimmer than in my experiment.

I think that 1/60th was already 2 stops darker than the ambient, so it didn't get any faster.

Check the ambient with the flash turned off. I bet it will be much lower than my 1/25th. This also has to do with the lens. If you don't have f/ 2.8 and are using say f/3.5, then the ambient shutter could be around 1/4th. Then two stops darker would only be 1/16th. Obviously if the shutter is limited to 1/60th it can't do anything.

That explains it. The ambient shutter of 1.5 sec was way too slow to be affected by your -2 ev change on the camera because the shutter is fixed at 1/60th as a minimum when shooting flash. You can make it longer, as you saw, but I have no idea why you would want to do that.

+2 ev over 1.5 sec is about 1/4th sec (divide 1.5 in half twice). This is obviously way below the 1/60th limit, so the 2 ev camera change will not affect your shutter.

So, the reason you can't duplicate my test is that your kit lens doesn't open up wide enough to allow a high enough shutter speed. However, your lens should open up to f/3.5 at the wide end (18mm I think), and in bright ambient light you might see the effect I mentioned.

In normal use, you won't be able to make you backgrounds bright without going to very low shutter speeds, and to do that you need to switch to either camera Manual or Shutter priority mode.

Try putting your camera in Manual mode with the aperture wide open and setting the shutter to 1/4th, I think you will get nice backgrounds. However, you might also get some ghosting, so you should select Rear Sync to place the ghost trails on the correct side of the motion.

Hi Russ,about compensation of exposition and brightness of background and subject,I've a question: does the zoom of the flash influence brightness of frame?Ie,I try to fire some shot at the same subject,aproximately at the distance of 3 foot,and changin' zoom of flash with the same camera zoom (set on 70mm),I observed that Histograms were similar from flash on 24mm to flash on 85mm;so flash set power differently for same subject with different zoom of flash?It means that if I use a zoom flash different from zoom camera I have always subject exposed correctly (TTL),naturally if subject is near the camera?And so is there any vantage use a different zoom between flash and camera?Maurizio

Zooming in on a subject does not change the distance to the subject, so the flash power should theoretically stay constant.

However, if you have read my blogs, you might remember that the flash sets its power based on the reflected energy of the monitor preflashes from the center of the frame. So TTL may set a slightly different flash power when you zoom in, because the subject occupies a larger portion of the frame, which can cause the reflected energy from the monitor preflashes to change slightly.

Zooming in will sometimes give a more accurate flash setting on the subject. Sometimes I zoom in, hit FV Lock, zoom back out to take the picture. This will allow you to avoid reflections from things that are nearer to the camera than the subject.

Also remember that every time you zoom, you have to refocus. Most lenses will not hold their focus as they are zoomed.

I have a question. When you adjust the FEC (flash exp comp) on the camera, it has no affect on the FEC on the flash itself. Shouldn't the camera talk to the flash and get it straight? If you do -1.0 on the camera and -1.0 on the flash unit, do you get -2.0?

Thanks Russ! What I am doing these days is I used the SB900, and have it in TTL mode and set the value on the screen (say -1.3 or whatever), then other times I incorporate one or two additional SB600's remotely, so I switch the 900 in master mode and it has a whole range of settings for the three flashes. So its kind of convenient because it remembers each FEC separately.

However I always sort of wonder if the TTL on each flash is really working and at times switch to manual mode and trial and error to get things right.

Thx Russ, for the brilliant masterpiece!I just come up a little question with FEC and expect to clarify my idea.

As the conclusion goes, FEC have the same effect as Camera EV adjustment.But it's obvious that flash would always lighten the subject rather than make them darker.So if I choose M-mode, how does I set a negative FEC make sense?

I'm not sure I completely understand your question, but I will answer what I think you are asking.

I think that you may have not have fully understood what I wrote.

The FEC button adjusts only the power of the flash.

The camera EC button adjusts the power of the flash, plus it adjusts the camera exposure (shutter and aperture) when using camera P,S, or A modes.

In camera M mode, the camera EC button changes the flash power exactly like the FEC button, but it does not affect the camera exposure (shutter and aperture) , because that is being set manually by the photographer.

When the FEC is set to negative numbers, it darkens the image FROM THE STANDARD BRIGHTNESS that is set by the flash exposure system when it is set to 0.0 ev.

Hi there, appreciate for your answer.I think I catch what your wrote,--When the FEC is set to negative numbers, it darkens the image FROM THE STANDARD BRIGHTNESS that is set by the flash exposure system when it is set to 0.0 ev.--Does THE STANDARD BRIGHTNESS mean 0.0 ev? Cause that is what in my idea.

As the quote goes, may I say thatnegative-FEC is equal to zero-FEC, with no flash occurs and exposures only by camera manual settings (shutter speed, F-stop).

In regular TTL mode: When you set 0.0 FEC (Flash Exposure Compensation) on the flash, it will fire the flash at a power that will illuminate the SUBJECT at 'standard brightness' assuming the ambient is zero. If the ambient is more than zero, then the flash and ambient will add and overexposure of the subject may result. This is why you must apply negative FEC unless you take the ambient out of the equation with the camera exposure controls (ISO, shutter, aperture).

Standard brightness is the brightness of the subject when a flash picture is taken at 0.0 FEC in a completely dark room.

The flash can only brighten an image. It cannot darken the image. Negative FEC will make the flash fire at a lower power than standard, but it will always fire and add to the ambient. For instance, -1.0 FEC will fire the flash at 1/2 the power from what it would have fired at 0.0 FEC.

When taking flash pictures, you should reduce the ambient portion of the exposure (using the ISO, shutter, aperture) by two to three stops, which effectively removes the ambient from the subject, and then the flash at 0.0 FEC brings the subject back to standard brightness.

Thanks for your instant answer!Maybe I totally misunderstood something!May I say FEC=0 is a value depends on standard brightness, or say the ambient light in P/A/S-mode, or camera exposure controls set by user in M-mode?

Or, FEC=0 is a value depends on 'standard brightness', which is a power of flash that can brighten a subject in a completely dark dark room as you said.

I think you may not understand that FEC only applies to the flash. It has nothing to do with the camera or the ambient.

There are two separate exposures for all flash pictures: one from the flash and one from the ambient. The two exposures are made on top of each other and add together.

The shutter, ISO, and aperture control the ambient exposure.

The TTL flash metering system controls the flash exposure.

FEC only affects the TTL metering system, which causes the power of the flash to change.

When you adjust the EC on the camera, it affects the ambient metering system and changes the shutter or aperture. Changes to EC are also sent to the flash metering system so that flash power will change up or down as the ambient exposure is changed..

When you adjust the FEC on the flash it does NOT adjust the EC of the camera.

When you set FEC=0 on the flash, the TTL metering will automatically adjust the flash power to expose the subject to 'standard' brightness.

And in M-mode with i-TTL,thought EC has no effect on camera exposure(say, shutter, ISO, aperture), but it can send information to the flash, adjust the power outputs, just like what FEC does.

I think all above is OK.

My main & beginning question is:

I choose M-mode & spot-metering,the built-in flash is set i-TTL.In this situation, the subject(in the center) will be light up to the correct exposure.And at this time, negative FEC/EC may cause subject darker.Darker than what? Darker than 'standard brightness'.And 'standard brightness' means the correct exposure as above?

Using a flash outdoors in high ambient light conditions is much more difficult than indoors in normal indoor artificial lighting.

You are exactly right. If you have a correct exposure set on the camera and you turn on your flash, it will overexpose the brightest areas. Even if you set the FEC to -3.0 FEC, it will still add to the ambient exposure (just a very little bit).

However, think about a situation where the background is bright and the subject is in the shade. In other words the subject is backlighted.

Now, if you set the camera for a correct exposure for the background, the subject will be dark. This is when you want fill flash.

This is also the situation that TTL-BL was made for. TTL-BL, will set the camera automatically for the background and add just the right amount of flash to make the brightness of the subject equal the background. You can use the camera in one of the auto modes, or you can use camera manual and use the built-in light meter.

However, for TTL-BL to work best always use Matrix metering - you can't even select TTL-BL if you use Spot metering.

Then, use the FEC to increase or decrease the brightness of the subject and the camera EC to increase or decrease the brightness of the whole image (or if in camera manual mode, just change the aperture or shutter to change the background brightness).

If you are using one of the camera automatic modes, and you adjust camera ec to darken the background by 1 ev, then you will have to brighten the flash by 1 ev to bring the subject back to standard brightness. If you are using camera Manual mode, changes to the aperture and shutter will not affect the flash power. The flash will always balance its power using the metering data, which is not affected by the shutter or aperture.

You must always be thinking when using flash outdoors. Never use fill flash if any direct sunlight is hitting the subject. The flash will add to it just as you said. Always try to get the subject in the shade and use flash to brighten the face.

""If you are using camera Manual mode, changes to the aperture and shutter will not affect the flash power. The flash will always balance its power using the metering data, which is not affected by the shutter or aperture.""

I take some photos for testing,I prefer changing the aperture or shutter WOULD affect flash power since the TTL metering system is depend on f-value, shutter speed and ISO.

The experiment I took is fixing 1/200s, ISO800, with f-value varies from f5.6 to f32.

And the centered subject is ALMOST have same brightness.(say, I did not examine it very carefully by checking histograms.)

Only for some cases the image will become darker that f-value form f22 to f32. I guess it results from the GN of my built-in flash is not enough power to lighten the subject.

I not sure, but I guess shutter speed, apertures may be related to the "through the lens" metering system.

About Me

I'm a retired Electrcal Engineer (MSEE USF 1975) now 11 years into my second career as a Photographer. I shoot mainly weddings, events, and portraits, but have also been doing some landscapes for fun.
I decided to start this blog to capture some of the many Nikon Speedlight posts I've made on http://Nikonians.org/forums . Of course, the main reason this is needed by the photography world is that Nikon, who makes the world's best Flashes, doesn't do a very good job of documenting them.
I am also a Commercial Pilot and an FAA Certificated Flight Instructor (CFII).